Problem: Which of these numbers is composite? ${13,\ 17,\ 27,\ 31,\ 73}$
Answer: A composite number is a number that has more than two factors (including 1 and itself). 13, 17, 31, and 73 each have only two factors. The factors of 27 are 1, 3, 9, and 27. Thus, 27 is the composite number.